This invention relates generally to clay and clay minerals, i.e., to crystalline layered lattice silicates having the general formula E.sub.i M.sub.x Si.sub.y O.sub.n (OH).sub.m, where M is Al, Mg, or Fe; x=2 to 6; y=2 to 8, n=2 to 20, m=0 to 8, and E.sub.i is one or more exchangeable ions (K, Na, Mg, Ca, Ba, Fe, Li, etc.). These layered silicates will hereinafter be referred to in this specification as "layered lattice silicates". More specifically, the invention relates to a method for nitriding materials of this type, including, e.g., kaolins and kaolinitic minerals, in order to produce products having new and unusual properties rendering same useful for a variety of purposes, including especially as fillers in plastic, rubber, ceramics and other systems.
Some modification of clays with nitrogenous compounds are known.
For example, organophilic organic-clay complexes are known which are dispersible in organic liquids to form a gel and thus are useful as gelling agents for paints and the like. They may be prepared by cation exchange with the sodium form of a smectite type clay, employing an organic cationic compound, preferably a quaternary ammonium salt, see Finlayson et al, U.S. Pat. No. 4,412,018. Such cation exchanges do not involve the silicon atoms of the clay.
C. A. Rice, U.S. Pat. No. 4,690,868 issued Sept. 1, 1987 describes the surface modification of clays, in particular kaolin, by a reaction with ammonia gas at temperatures below 1000.degree. C., preferably in the temperature range of about 700.degree. C. to 900.degree. C., to form bound NH.sub.2 groups on the surface. The surface aminated products are particularly useful as fillers in resin or plastic systems.
Silicon nitride is a known compound and is described in Grant and Hackh's Chemical Dictionary as Si.sub.3 N.sub.4, a white powder insoluble in water, very resistant to thermal shock and chemical reagents.
An article by R. K. Brow and C. G. Pantano in J. Am. Ceram. Soc., vol. 70(1) 9-14 (1987), reports an experimental study of the nitridation of microporous silicon dioxide thin films in ammonia. Sol-gel-derived silicon dioxide appeared to be an effective precursor material for the development of silicon oxynitride thin films by a high-temperature reaction with ammonia. Because of their initial microporosity, these films are rapidly incorporated with high concentrations of nitrogen (up to 40 mol %) to produce a more compositionally homogeneous material than is formed when thermally grown silicon dioxide is nitrided. The chemical structure of the film was monitored as a function of treatment temperature using X-ray photoelectron spectroscopy, which showed that the nitrogen converted from amine to nitride species as the treatment temperature exceeded 800.degree. C. The utilization of silicon nitride and silicon oxynitride films in microelectronic device application is mentioned.
An article by Sang Wook Choi et al in the Journal the Korean Ceramic Society, vol. 23(5) 17-24 (1986) published on Oct. 30, 1986, describes the synthesis of products named .beta.' sialon from compositions containing kaolin, graphite and silicon or aluminum at temperatures in the range of 1300.degree.-1450.degree. C. under a flowing gas of 90% N.sub.2 -10% H.sub.2 for 20 hours. A composition of kaolin and graphite was also treated. The graphite functions as a reducing agent. The reactions are termed carbothermal reduction-nitridation.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a relatively simple, inexpensive, and effective process, which enables surface modification of a clay mineral, such as a kaolin clay, by surface nitride groups, thereby providing a new product having valuable application, e.g., as a filler for paints, plastics, rubbers, paper, ceramics and possibly microelectronics.
It is a further object of the present invention, to provide a process yielding products which enable the production of filled systems, for example, resin-filled systems incorporating the products of the invention, which have superior properties with respect to mechanical aspects of same, as compared to prior art filled systems based upon prior art kaolin clay, and similar fillers.